Hi Will,
I read through the copy of Timber Frame Joinery and Design Volume 2 and can understand your impression. My initial takeaway is that I find some of the nomenclature surrounding square rule to be inconsistent and unsatisfactory.
For example, as you cited, the article by Anderson from Dec. 92 describes the Japanese approach as "the center line square rule method". However, the very definition of square rule in "Introduction to Layout" is as follows "The square rule was developed in North America, apparently near the turn of the 19th century (the earliest dated example is 1803). Timber framers could now get roughly squared timbers relatively easily. The trees here were large and could be milled or hewn down to a nearly consistent section along the length without concern for wasting too much of the tree, and surfaces were true and straight enough to serve as reference." I feel that the above definition is pretty much the stock basic definition of square rule as far as I have encountered and I think that most people would be familiar with, with notable emphasis to North American development as well as the edge as primary and preferred reference face. The eastern systems of layout clearly predate and are are on a totally separate line of development from anything North American, so I think that we are setting up for a lot of confusion in trying to refer to the two as being part of the same basic system.
I know that there are techniques for snapping lines in square rule (and scribe rule as well!) but I would argue that these are not central to the layout method in the way that a centered snap line is central in Eastern layout. Edge reference is the norm and the gold standard in square rule, snapping lines typically seems to be a way to manage unusual or undesirable situations in practice. Square rule has a mindset that revolves around the edge of the timber, as is seen in the practice of offsetting tenons a given distance from the edge. The same article by Anderson speaks to the centrality of the snapped center line to Japanese layout. “Generally, the carpenter uses four different classes of lines to lay out
his work. These are the shinzumi,suihezumi (or mizu), tatezumi and the yorizumi.”...”Shinzumi can be read as “center line” but a different reading of the Chinese characters is “true line”, which comes closer to how the mark is actually regarded while constructing the rest of the house”. In the end, I think that the designation of square rule regarding Eastern practice is inadequate to describe a system so different from what most people would readily recognize as “square rule”. Eastern layout systems have their own very definite, developed, and distinct history, practice, tooling, and philosophy. Surely they deserve the recognition of those distinctions to be acknowledged by the nomenclature used in reference.